Prior to the invention, a device was known which had a weight c attached to the upper side of an after-fulcrum rear end b of a key a, as shown in FIG. 9. This weight provided the key with return force. Another device was known which had one end of a U-shaped plate spring d fastened to the after-fulcrum rear end b of the key a and the other end of the U-shaped plate fastened to a chassis e as shown in FIG. 10, so as to provide a return force on the key a. Either of these prior art devices provides a key touch load versus key stroke characteristic similar to that of a piano key.
With such a conventional device as shown in FIG. 9 or FIG. 10, the static characteristics of the key a comprising the key stroke versus key touch load characteristic when the key a is slowly depressed and when the key a is slowly returned to its original position do not constitute a hysteresis characteristic as shown in FIG. 11, so that the key rises if the pressing force exerted by a finger is weakened even slightly as the key a is being further depressed. This aspect tends to cause shaky depressing of the key a and makes it difficult to achieve a delicate expression in a performance as done by a piano touch.
In the case of the conventional device shown in FIG. 9, the return force exerted on key a is derived only from the weight c, so that the mass of the keyboard as a whole is increased. If the mass of the weight c is decreased, the after-fulcrum portion of the key must be made longer in order to be well-balanced with the initial load of the key a, thus resulting in the disadvantage of a relatively larger-sized keyboard.
With the conventional device shown in FIG. 10, the touch load of the dynamic characteristic of the key a is small due to the small equivalent mass of the key a as shown in FIG. 11, which gives rise to the inconvenience that a player of the instrument cannot feel the reaction of the key as felt with a piano when striking a key. There is also another inconvenience that the plate spring has to be removed before the key a can be removed and that a skilled hand is necessary for detaching and attaching the plate spring.